This invention relates to hydrometallurgical processing of precious metal sources and, more particularly, to cyanic extraction of precious metal from refractory precious metal sources including ores, concentrates, and tailings.
As used herein, the term "precious metal" means gold, silver and platinum and the term "precious metal ore" includes precious metal concentrates and tailings derived from precious metal ore.
The cyanide process, also known as cyanidation, has long been used to extract gold from free milling gold ore. In the application of the cyanide process, a soluble, gold-cyanide complex is obtained by agitationally treating comminuted, free milling gold ore with an aerated, alkaline solution containing a low concentration of water soluble cyanide. In this process, aurum (gold) interacts with, for example, sodium cyanide in the presence of oxygen and water to produce sodium auricyanide (gold-cyanide complex) and sodium hydroxide.
In contrast to the free milling ores which are readily amenable to cyanidation without prior chemical conditioning, it is well known in the hydrometallurical art that other precious metal ores offer varying degrees of resistance to direct cyanidation. This resistance is manifested in some instances by unusually long extraction time and in other instances through "consumption" of cyanide and oxygen which results in reduced yield of solubilized gold.
Precious metal ores which resist cyanidation such as pyritic gold ore, arsenical gold ore and quartzitic gold ore are referred to in the art as refractory ores. A pyritic ore comprises metallosulfide as, for example, iron sulfide such as iron monosulfide and iron disulfide; an arsenical ore is typified by arsenopyrite which is chemically expressed as iron sulfarasenide; and a quartzitic ore comprises crystalline silicon dioxide. Sulfides of copper, iron, antimony and arsenic have been characterized as cyanicides because of their significant adverse effect on cyanide in respect of the degree of gold solubilization. It appears that various sulfide compositions have solubility characteristics which permit interaction with cyanide and oxygen to form, for example, ferrocyanide, cyanate and thiocyanate, all to the diminution of maximum gold extraction and recovery.
In order to place refractory precious metal ores in suitable form for effective and efficient cyanidation, it has been the practice to condition such ores prior to cyanidation by subjecting the ores to appropriate treatment. Examples of pre-cyanidation conditioning include (a) treatment of the ore with lime and air, (b) treatment of the ore with a chemical oxidizing system containing calcium hypochlorite, and (c) treatment of the ore by roasting. These procedures for effecting pre-cyanidation treatement are discussed in the Monograph entitled Gold And Silver Cyanidation Plant Practice by F. W. McQuiston and R. S. Shoemaker, The American Institute of Mining, Metallurical and Petroleum Engineers, Inc., 1975. It is suggested that the purpose of the conditioning step is to unlock or liberate the precious metal from the refractory ore and to covert any refractorizing constituent into a form which does not consume cyanide and/or oxygen.
Since prolonged extraction time and pre-cyanidation condition impose economic and technological burdens in the processing of refractory precious metal ore, it would be significantly advantageous to provide a method for effecting cyanic extraction of precious metal from refractory ore which would substantially increase the precious metal extraction rate and would eliminate the necessity for a pre-cyanidation conditioning step.